In warnings, say "previous declaration" rather than "first

In warnings, say "previous declaration" rather than "first
declaration".
* src/symtab.c (redeclaration): Do that here.
* src/reader.c (record_merge_function_type): In the case of a result
type clash, report the previous declaration rather than the very first
one in the grammar file.
* tests/glr-regression.at (Missed %merge type warnings when LHS type is
declared later): Add a third declaration to check this behavior.
* tests/input.at (Incompatible Aliases): Update output.